Desert Wildflower Blooms

Spring Flowering Season

The spring flowering season in the Arizona
Upland subdivision spans from mid February to mid June with a peak from
mid March to late April depending on rainfall and temperatures during the growing
season. In the warmest areas of the Lower
Colorado River Valley subdivision it is normally a couple of weeks earlier,
though it sometimes starts as early as November. The different life forms which
dominate at different times vary in their showiness and reliability. The early-blooming
winter annuals can create an incredible display, but do so only rarely. Later-blooming
species bloom more dependably, but mostly not in great masses of color. The progression
of spring bloom described below is for average years near Tucson. It may be three
weeks earlier or later depending on weather, elevation, and latitude.

Winter Annuals

Winter annuals such as poppies (Eschscholtzia mexicana), lupines (Lupinus
sparsiflorus and others), and owl clover (Castilleja exserta, syn. Orthocarpus
purpurascens) create the vast carpets of color for which the Sonoran and
Mohave deserts are so famous. This event may occur between late February and
mid April, usually in mid March. Annuals are highly dependent on rainfall. Massive
and widespread displays occur only about once a decade, when the winter rainy
season is both earlier and wetter than normal. Good shows happen in localized
areas every three or four years. A good bloom cannot be reliably predicted more
than a week or two before it begins (see next page for details), and it lasts
at peak beauty for no more than two weeks. Seeing one requires being able to
travel on short notice, and perhaps great distances. Death Valley may be spectacular
in a year when Organpipe Cactus National Monument is poor. The high Mohave Desert
may peak two or three weeks later than the lower elevation and more southerly
Sonoran Desert. A good bloom may occur in a remote area and remain undiscovered.

Herbaceous perennials and small shrubs such as penstemon (especially Penstemon
parryi), brittlebush (Encelia farinosa), and fairy duster (Calliandra
eriophylla) also require rain to bloom but are less sensitive to its timing.
They are somewhat more dependable than the annuals, making a good show in about
half of the years and peaking some time in March. These species usually grow
as individuals or in small patches and do not create masses of color.

Cacti, because they store water, are fairly independent of rain. They bloom
well nearly every year though wetter years produce more flowers. The greatest
diversity of spring-blooming species can be seen in April. The cactus show continues
as the abundant prickly pears bloom in early May, followed by saguaros from mid
May to mid June.

Trees and large shrubs are fairly dependable bloomers, though flowers will
be sparse in dry years. Creosote bush (Larrea tridentata) and whitethorn
acacia (Acacia constricta) both bloom mainly in spring and sometimes again
in summer. Blue palo verdes (Cercidium floridum) turn bright yellow in
late April, followed two weeks later by the much more abundant but paler yellow
foothill palo verdes (C. microphyllum). Desert ironwood trees (Olneya
tesota) bloom heavily about every other year with masses of lavender flowers,
usually in late May. The abundant ocotillo reliably produces spikes of red flowers
through-out April. These species bloom about two weeks earlier in western Arizona.

Summary

If you want to see the famous carpets of color, keep abreast of local news
from Palm Springs to Tucson and from Death Valley to northern Mexico. Begin checking
in January lest you miss an early show. (The dunes can sometimes begin in November,
but this catches nearly everyone off guard so it's pure luck to discover such
an early show.) You'll find masses of annuals somewhere in this area about once
every three or four years. If you want dependability and will settle for less
quantity, success is almost guaranteed in the middle half of April.

The summer flowering season season
begins a few weeks after the first summer rain and continues into late fall.
Though there are many beautiful species to be seen, there are rarely massive
displays of color in this season, because the summer rains are more sporadic
and localized than the winter rains and the soil dries rapidly in the heat.

Summer annuals such as summer poppy (Kallstroemia grandiflora) and
devil's claw (Proboscidea parviflora) germinate within a few days after
the first soaking summer rain and begin to flower as soon as three weeks later.
Chinchweed (Pectis papposa) is the most widely-adapted summer annual;
it ranges from New Mexico into the central Mohave Desert where it is the only
summer annual (summer rains are uncommon in the Mohave). It can form showy carpets
of yellow when rains are abundant.

Herbaceous perennials and small shrubs bloom opportunistically if they get
enough rain. Trailing four-o'clock (Allionia incarnata) and desert marigold
(Baileya multiradiata) are nonseasonal, flowering in response to rain
in all but the coldest months. Fairy duster will also bloom again in wet summers,
but not as profusely as in spring. Sacred datura (Datura wrightii) is
mainly a summer perennial though it may begin flowering as early as April in
warmer areas. There are several woody shrubs that bloom in late fall. Most are
composites such as burroweed (Isocoma tenuisecta) and desert broom (Baccharis
sarothroides). Desert senna (Senna covesii) and Coulter hibiscus (Hibiscus
coulteri) flower in response to any warm rain and peak in summer when most
such rain occurs. Desert zinnia (Zinnia pumila) is truly biseasonal, flowering
well in both rainy seasons.

Cacti include several summer-flowering species. The pincushion cactus Mammillaria
grahamii microcarpa makes buds during its previous growing season, then goes
dormant during the dry season. The buds burst into bloom five days after each
of the first two or three summer rains. The fishhook barrel cactus (Ferocactus
wislizeni) is much larger thn the pincushion and less dependent on rain;
it flowers throughout August and September.

Trees and large shrubs are nearly all spring bloomers, but a few bloom again
in summer if rains are generous. Whitethorn acacia (Acacia constricta),
velvet mesquite (Prosopis velutina) flower heavily in spring and often
again in summer. Desert willow (Chilopsis linearis) flowers from spring
through fall if it has enough water.

Summary

Though the Sonoran Desert has two flowering peaks, there is almost always
something in bloom. The only exceptions are after a hard winter freeze or during
severe droughts.